Gold Filled is two sheets of gold usually layered over a sheet of brass that is then squeezed and rolled to the desired thickness.
On some watches the Rolled or Filled gold watch case may be backed up by a 10, 20, 25 year or more symbol/stamp, this means that the thickness of the layered gold sheet is warranted to last more than the stamped amount of years before it will eventually wear through to the brass layer underneath.
so if you see a watch that is layered and doesn't have a warranted year stamp but it may say layered then i would only consider it slightly thicker than heavy plated material and would not waste my time on it unless you got it for the right price that is. As far as how thick say a 25 year warranted layering is i'm not sure about.
See example of case ive attached.